Display panel comprising a light guide

ABSTRACT

The display panel  21  comprises a light guide plate  2,  a second plate 4 and a movable element  3  between both plates, for locally coupling light out of the light guide plate  2.  Voltages applied to electrodes  5, 6  and  25  locally bring the movable element  3  into contact with the light guide plate  2  or the second plate  4.  Means for reducing sticking of the movable element  3  to the plates comprise a roughness R 1  at the contact surface  14  of the light guide plate  2  facing the movable element  3,  a roughness R 2  at the contact surface  15  of the movable element  3  facing the light guide plate  2,  a roughness R 3  at the contact surface  16  of the second plate  4  facing the movable element  3  and a roughness R 4  at the contact surface  17  of the movable element  3  facing the second plate  4.  The roughnesses R 1  and R 2  are smaller than at least one of the roughnesses R 3  and R 4.  By virtue thereof, sticking is reduced and the display panel  21  requires relatively little energy to be operated.

The invention relates to a display panel comprising a light guide plate,a second plate facing the light guide plate, a movable element betweenthe light guide plate and the second plate, for locally coupling lightout of the light guide plate, the light guide plate having a contactsurface facing the movable element for contacting the movable element,the second plate having a contact surface facing the movable element forcontacting the movable element, the movable element having a firstcontact surface facing the light guide plate for contacting the lightguide plate and a second contact surface facing the second plate forcontacting the second plate, means for reducing sticking of the movableelement to the plates comprising a roughness R1 at the contact surfaceof the light guide plate and a roughness R2 at the first contact surfaceof the movable element, at least one of the roughnesses R1 and R2 beinglarger than 5 nm and both the roughnesses R1 and R2 being smaller than100 nm, and electrodes to locally bring the contact surfaces of themovable element into contact with respectively the contact surface ofthe light guide plate and the contact surface of the second plate.

An embodiment of the display panel of the type mentioned in the openingparagraph is known from WO 99/28890.

The known display panel comprises a light guide plate of a fluorescentmaterial, in which display panel, in operation, light is generated andtrapped so that this first plate forms a light guide, a second plateand, between said two plates, a movable element. By applying suitablevoltages to the electrodes on the light guide plate, the second plateand the movable element, the movable element is locally brought intocontact with the light guide plate or the second plate. The electrode onthe movable element is also referred to as common electrode. Atlocations where the movable element is in contact with the light guideplate, light is coupled out of the light guide plate during operationand scattered out of the movable element. Locations where light can becoupled out of the light guide plate are known as picture elements. Byregulating whether or not light is coupled out at picture elements animage is represented.

Roughnesses R1 and R2 are present at the contact surface of the lightguide plate facing the movable element and at the contact surface of themovable element facing the light guide plate to reduce sticking of themovable element to the light guide plate by reducing adhesive Van derWaals' forces. Roughness is defined as the distance range within whichthe surface profile is confined, as measured with e.g. an atomic forcemicroscope. It is known that, if at least one of the roughnesses R1 andR2 is larger than 5 nm, the Van der Waals' forces are reducedsubstantially. On the other hand, if one of the roughnesses R1 and R2 islarger than 100 nm, then the optical contact between the movable elementand the light guide plate is reduced, such that the coupling of lightout of the light guide plate is severely diminished. Furthermore,anti-adhesion layers are present in between the movable element and thelight guide plate as well as in between the movable element and thesecond plate. These layers reduce sticking of the movable element to theplates by reducing the magnitude of the chemical adhesive forces.

Electrodes on the surfaces of both plates facing the movable element areseparated from the common electrode on the movable element by aninsulating layer having a thickness of about one micrometer. Theseinsulating layers are present on the electrodes on the light guide plateand the second plate to prevent direct electrical contact with thecommon electrode on the movable element. The insulating layer representsthe contact surface of the plate. Therefore, the contact surface of amember facing another member denotes the free surface of the member,whether or not covered with layers, which is capable of making directcontact with the other member.

A drawback of the known display panel is that it requires relativelymuch energy to locally bring about contact between the light guide plateand the movable element by applying voltages to the electrodes.Therefore, in operation, the known panel requires relatively muchenergy.

It is an object of the invention to provide a display panel of the kindmentioned in the opening paragraph, which requires relatively littleenergy to be operated.

This object is achieved in that the means for reducing sticking furthercomprise a roughness R3 at the contact surface of the second plate and aroughness R4 at the second contact surface of the movable element, andboth the roughnesses R1 and R2 are smaller than at least one of theroughnesses R3 and R4.

The inventors have recognized that the energy to locally bring aboutcontact between the light guide plate and the movable element byapplying voltages to the electrodes is smaller if a roughness R3 ispresent at the contact surface of the second plate facing the movableelement and a roughness R4 is present at the contact surface of themovable element facing the second plate. If at least one of theroughnesses R3 and R4 is larger than 5 nm, the Van der Waals' forcesbetween the second plate and the movable element are reducedsubstantially, resulting in a display panel requiring relatively littleenergy to locally bring about contact between the light guide plate andthe movable element. Taking into account only these Van der Waals'forces, there is no reason for the roughnesses R3 and R4 to be over 5nm. However, the inventors have further recognized that the roughnessesR3 and R4 influence, at a given voltage difference between the secondplate and the movable element, the attractive electrical force betweenthe second plate and the movable element. This attractive electricalforce is inversely proportional to the square of the distance betweenthe electrode on the second plate and the common electrode on themovable element. For the second plate and the movable element, whichstick together tightly via intimate surface contact, this attractiveelectrical force is relatively large. To pull them apart, thisrelatively large attractive electrical force has to be overcome, as itis very difficult in practice to delete this attractive electrical forceby removing the voltage difference. If the distance between the secondplate and the movable element is larger, the movable element and thesecond plate do not stick together tightly via intimate surface contact,i.e. the contact surfaces mostly touch each other at the tips ofprotrusions protecting from either one of the surfaces or from bothsurfaces, and it is easier to pull them apart. It is clear that if themovable element sticks tightly to the second plate, the voltagedifference between the electrodes on the movable element and the lightguide plate that is necessary to interrupt this contact is relativelylarge. By virtue of the roughness R3 at the contact surface of thesecond plate facing the movable element and the roughness R4 at thecontact surface of the movable element facing the second plate, asmaller attractive electrical force between the movable element and thesecond plate, if the movable element is in contact with the secondplate, has to be overcome in order to interrupt this contact. Thisattractive electrical force can still be non-negligible if one of theroughnesses R3 and R4 is smaller than 5 nm and the other roughness isequal to 5 nm. A roughness larger than 5 nm causes the energy necessaryto interrupt this contact to be smaller. In fact, the maximum value forthe roughnesses R3 and R4 is limited by the space in between the lightguide plate and the second plate, which space is reduced by thethickness of the movable element.

A relatively small attractive electrical force between the movableelement and the second plate, induced by voltages applied to theelectrodes on the movable element and the second plate if the movableelement is in contact with the second plate, causes the requiredelectrical force, induced by voltages applied to the electrodes on themovable element and the light guide plate, necessary to interrupt thiscontact to be relatively small, relatively little subject to variationsand well-controllable.

In an embodiment of the display panel in accordance with the invention,at least one of the roughnesses R3 and R4 is larger than 100 nm and bothroughnesses R3 and R4 are smaller than 1000 nm.

The electrodes on the surface of both plates facing the movable elementare covered by an insulating layer having a thickness of about onemicrometer. This insulating layer represents the contact surface of theplate. For a roughness below 100 nm, the electrical forces to beovercome are hardly reduced as the roughness is small compared to thethickness of the insulating layer. For larger values of the roughness,the electrical force to be overcome is reduced significantly. If theroughness is larger than 1000 nm, the roughness is non-negligiblecompared to the distance of about four micrometers between both plates,which distance is reduced by the thickness of the movable element, whichis generally in the range between one and two micrometers.

In another embodiment of the display panel in accordance with theinvention, the roughness R2 is larger than the roughness R1 and theroughness R4 is larger than the roughness R3. Both the light guide plateand the second plate have smooth contact surfaces, which is easier torealize in a production process. Furthermore, the light guide propertiesof the light guide plate improve with increasing smoothness of itssurface.

In a modification of the last embodiment, the roughness R2 is in therange between 5 and 100 nm and the roughness R4 is in the range between100 and 1000 nm. These are the preferred ranges for the roughnesses nowpresent on the movable element. Furthermore, as the roughness R4 rangesbetween 100 and 1000 nm, the isotropicity of the emitted light at thepicture element is large.

It is favorable if at least one of the roughnesses R2 and R4 is broughtabout by inorganic protrusions from the movable element. The inorganicprotrusions are not readily subject to elastic and/or plasticdeformations when the movable element is in contact with one of theplates, thus preventing elastic and/or plastic deformation-induced Vander Waals' adhesion. It is especially favorable if the movable element,having a volume, comprises a matrix layer selected from the groupconsisting of glassy amorphous and crystalline polymeric layers, with 1to 25 percent of the volume being occupied by TiO2 particles having adiameter between 200 and 400 nm, and an indium tin oxide layer. Thematrix layer exists either in a glassy amorphous state or in acrystalline state or in a mixed glassy amorphous/crystalline state suchas to give the polymeric material a stiffness similar to that of acrystalline organic material. These matrix layers are for exampleparylene, polymethylmethacrylate, some fluoropolymers and polyimidelayers. These materials do not readily suffer from plastic deformationsand/or creep. The movable element may have a surface roughness on thesurface facing the light guide plate in the range from 30 to 50 nm, asformed by TiO2 particles slightly protruding from the movable elementand a surface roughness of the surface facing the second plate in therange from 100 to 1000 nm. The thickness of the movable element may befor example in between one and two micrometers. The conducting indiumtin oxide (ITO) layer represents the common electrode across the entiresurface of the movable element in order to apply a voltage to themovable element. The ITO layer is transparent to prevent absorption oflight emitted at the contact surface between the light guide plate andthe movable element. A preferred thickness of the ITO layer is about 30to 50 nm. Light that is extracted from the light guide by the movableelement is scattered by scattering centers, such as TiO2, and thesurface roughnesses on both sides of the movable element. Particlessmaller than 0.05 micrometer do not scatter the light very efficiently.Particles larger than one micrometer are too large to be properlyconfined within the movable element.

In an embodiment of the display panel in accordance with the invention,the display panel is part of a display device. The display devicefurther comprises selection means arranged to apply voltages to theelectrodes dependent on the image information to be displayed.

These and other aspects of the invention will be further elucidated anddescribed with reference to the drawings, in which:

FIG. 1 shows schematically a cross-sectional view of the display panel,

FIG. 2 shows schematically a part of the display panel,

FIG. 3 shows schematically details of a part of the display panel,

FIG. 4 shows schematically the movable element, and

FIG. 5 shows schematically the display device.

The figures are schematic and not drawn to scale and in all the figuresidentical reference numerals refer to corresponding parts.

In FIG. 1, the display panel 21 comprises a light guide plate 2, amovable element 3 and a second plate 4. Electrodes 5 and 6 are arranged,respectively, on the sides of the light guide plate 2 and the secondplate 4 facing the movable element 3. The display panel 21 comprises acovering element 7 connected to the light guide plate 2, thus forming aspace 8. The display panel 21 further comprises a light source 9. Lightgenerated by the light source 9 is coupled into the light guide plate 2.The light travels inside the light guide plate 2 and, due to internalreflection, cannot escape from the light guide plate 2 unless thesituation as shown in FIG. 2 occurs. FIG. 2 shows the movable element 3locally lying against the light guide plate 2. In this state, part ofthe light enters the movable element 3. The movable element 3 scattersthe light, so that it leaves the display panel 21. The light can issueat both sides or at one side. Preferably, in order to cause the light toexit the movable element 3, the movable element 3 comprises a layer of amaterial, for instance a polymer, also called a matrix layer, in whichscattering centers are present. Such scattering centers may be formed byparticles of a material other than the material of the matrix layer orby bubbles in the matrix layer. Light entering the movable element 3 isscattered by scattering centers. In FIG. 2, this is indicated by meansof straight arrows. Furthermore, in FIG. 2 also the contact surface 14of the light guide plate 2 facing the movable element 3, the contactsurface 15 of the movable element 3 facing the light guide plate 2, thecontact surface 16 of the second plate 4 facing the movable element 3,and the contact surface 17 of the movable element 3 facing the secondplate 4 are indicated.

In FIG. 3, the movable element 3 is positioned between the light guideplate 2 and the second plate 4 by means of spacers 12 and 13. A commonelectrode 25 is part of the movable element 3. The common electrode 25can be present on the side of the movable element 3 facing the lightguide plate 2, on the side of the movable element 3 facing the secondplate 4 or may even be part of the bulk of the movable element 3.Electrodes 5 and 6 are covered by insulating layers 10 and 11 in orderto preclude direct electric contact between the common electrode 25 onthe movable element 3 and the electrodes 5 and 6. The insulating layer10 on the light guide plate 2 is not necessary if the common electrode25 is on the side of the movable element 3 facing the second plate 4. Byapplying suitable voltages to the electrodes 5, 6 and the commonelectrode 25 on the movable element 3, an electric force F is generatedwhich presses the movable element 3 against the electrode 5 on the lightguide plate 2. The electrode 5 is transparent and may consist of ITO.The contact between the movable element 3 and the light guide plate 2causes light to leave the light guide plate 2 and enter the movableelement 3 at the location of contact. In the movable element 3, thelight is scattered and leaves the display panel 21 via the transparentelectrode 5 and the light guide plate 2. The roughness R1 at the contactsurface 14 of the light guide plate 2 facing the movable element 3 andthe roughness R2 at the contact surface 15 of the movable element 3facing the light guide plate 2 are present to reduce sticking of themovable element 3 to the light guide plate 2. At least one of theroughnesses R1 and R2 is larger than 5 nm and both roughnesses R1 and R2are smaller than 100 nm. A roughness R3 at the contact surface 16 of thesecond plate 4 facing the movable element 3, and a roughness R4 at thecontact surface 17 of the movable element 3 facing the second plate 4,both roughnesses R1 and R2 being smaller than at least one of theroughnesses R3 and R4, reduce sticking of the movable element 3 to thesecond plate 4. Preferably, at least one of the roughnesses R3 and R4 islarger than 100 nm and both roughnesses R3 and R4 are smaller than 1000nm. If the roughness R2 is larger than the roughness R1 and theroughness R4 is larger than the roughness R3, both the light guide plate2 and the second plate 4 may have smooth contact surfaces facing themovable element 3, which is easier to realize in a production process.In particular, the preferred range of roughnesses is present at thecontact surfaces of the movable element 3 facing the light guide plate 2and the second plate 4: the roughness R2 is in the range between 5 and100 nm and the roughness R4 is in the range between 100 and 1000 nm.

In FIG. 4, the matrix layer 18 is a glassy amorphous layer. The matrixlayer 18 may also be a crystalline polymeric layer. Examples of theselayers are a parylene, polymethylmethacrylate, fluoropolymer andpolyimide layer. Also a cross-linked polymer layer with mechanicalproperties equivalent to those of a glassy amorphous or crystallinepolymeric layer can be used. The thickness of the matrix layer 18 ispreferably between 0.5 and 3 micrometer, and most preferably between 1to 2 micrometer. The scattering particles 19 also act as inorganicprotrusions 24 and are made of for instance TiO2, BN, ZrO2, SiO2, Si3N4and Al2O3. The average size of the scattering particles 19 is preferablybetween 200 and 400 nm. The concentration of scattering particles 19 isin the range from 1 to 50 percent. Preferably the concentration is inbetween 1 and 25 percent. Preferably the difference in refractive indexbetween the matrix layer 18 and the scattering particles 19 is largerthan 0.1. For smaller differences the scattering efficiency of thescattering particles 19 is very low. Good scattering results areobtained when the refractive index is larger than 0.5. Preferredmaterials for the scattering particles 19 are TiO2, BN, and Al2O3, sincethese materials are practically colorless. The refractive index of thematrix material 18 is preferably close to the refractive index of thematerial of the light guide plate 2, i.e. the difference is less thanapproximately 0.2. In this case, the reflection at the contact surfacebetween the light guide plate 2 and the movable element 3 is small. Aconducting indium tin oxide layer 20 is present to apply a voltage tothe movable element.

In FIG. 5, the display device 1 comprises the display panel 21 andselection means 22, arranged to apply voltages to the electrodes 5, 6and 25 dependent on the image information to be displayed.

1. A display panel (21) comprising a light guide plate (2), a secondplate (4) facing the light guide plate (2), a movable element (3)between the light guide plate (2) and the second plate (4), for locallycoupling light out of the light guide plate (2), the light guide plate(2) having a contact surface (14) facing the movable element (3) forcontacting the movable element (3), the second plate (4) having acontact surface (16) facing the movable element (3) for contacting themovable element (3), the movable element (3) having a first contactsurface (15) facing the light guide plate (2) for contacting the lightguide plate (2) and a second contact surface (17) facing the secondplate (4) for contacting the second plate (4), means for reducingsticking of the movable element to the plates (2, 4) comprising aroughness R1 at the contact surface (14) of the light guide plate (2)and a roughness R2 at the first contact surface (15) of the movableelement, at least one of the roughnesses R1 and R2 being larger than 5nm and both the roughnesses R1 and R2 being smaller than 100 nm, andelectrodes (5, 6, 25) to locally bring the contact surfaces (15, 17) ofthe movable element into contact with respectively the contact surface(14) of the light guide plate (2) and the contact surface (16) of thesecond plate (4), characterized in that the means for reducing stickingfurther comprise a roughness R3 at the contact surface (16) of thesecond plate (4) and a roughness R4 at the second contact surface (17)of the movable element (3), and both roughnesses R1 and R2 are smallerthan at least one of the roughnesses R3 and R4.
 2. A display panel (21)as claimed in claim 1 characterized in that at least one of theroughnesses R3 and R4 is larger than 100 nm and both the roughnesses R3and R4 are smaller than 1000 nm.
 3. A display panel (21) as claimed inclaim 1, characterized in that the roughness R2 is larger than theroughness R1 and the roughness R4 is larger than the roughness R3.
 4. Adisplay panel (21) as claimed in claim 3, characterized in that theroughness R2 is in the range between 5 and 100 nm and the roughness R4is in the range between 100 and 1000 nm.
 5. A display panel (21) asclaimed in claim 3, characterized in that at least one of theroughnesses R2 and R4 is brought about by inorganic protrusions (24)from the movable element (3).
 6. A display panel (21) as claimed inclaim 5, characterized in that the movable element (3), having a volume,comprises a matrix layer (18) selected from the group consisting ofglassy amorphous and crystalline polymeric layers, with 1 to 25 percentof the volume being occupied by TiO2 particles (19) having a diameterbetween 200 and 400 nm, and an indium tin oxide layer (20).
 7. A displaydevice (1) comprising the display panel (21) of claim 1.